1. Field of the Invention
The present invention relates to displacement sensors, more specifically, to displacement sensors employing the phenomenon of giant magnetoresistance.
2. The Prior Art
Linear and angular displacement sensors are used for high-precision machining and measuring, for manufacturing and testing components with very tight dimensional tolerances. These devices are typically coupled to lathes, presses, robot arms, etc.
There are several types of displacement sensors in current use. One type of sensor measures the inductance of a metal bar passing through a wire coil. The drawbacks of this type of sensor include limited precision, large size, lack of portability, sensitivity to vibrations, expense of fabrication, and the need for large amounts of power to operate.
A second type of sensor is based on the magnetoresistive effect. Essentially, the electrical resistance of some materials is affected by the strength of local magnetic fields. This type of sensor uses the distance between a magnet and a strip of these magnetoresistive materials to measure distance. The main drawback of this type of sensor is that the change in resistance due to a change in the magnetic field strength is very small relative to the overall resistance of the element. Thus, small differences are difficult to detect and can be affected by other environmental conditions such as temperature and humidity.
A third type of sensor is described in U.S. Pat. No. 5,475,304, issued on Dec. 12, 1995 to Prinz, Hubbard, and Parsons, and entitled MAGNETORESISTIVE LINEAR DISPLACEMENT SENSOR, ANGULAR DISPLACEMENT SENSOR, AND VARIABLE RESISTOR USING A MOVING DOMAIN WALL. This patent discloses a sensor with a single element operating under the giant magnetoresistive (GMR) effect, as described below, and a means for inducing a magnetic domain wall in the element. The sensor of Prinz et al. measures the change in resistance of a single element due to a change in location of the domain wall. This sensor is an improvement over the above-described sensors of the prior art, but still has drawbacks, including a sensitivity to environmental conditions, such as temperature and humidity, and a sensitivity to "snagging", as described below.